This invention relates to an exhaust sampling system that can be used to obtain diluted samples of exhaust gas to, for example, measure particulate matter.
Exhaust sampling systems are used to obtain a sample of an exhaust gas for measurement of various properties of the exhaust gas. In one example, an exhaust gas sample is obtained from the tailpipe of a vehicle's engine, and the exhaust sample is diluted to a desired dilution ratio. A typical arrangement utilizes a single dilution gas flow device to provide a diluent such as air to a mixer of the sampler, which mixes the exhaust gas sample and the diluent to form a diluted exhaust gas. A typical exhaust sampling system can achieve a dilution ratio of 50:1 while maintaining a desired accuracy.
For the above described system, the flow of diluent is controlled and measured using a single dilution gas flow device, such as a mass flow controller. The flow of diluted exhaust gas is measured using a flow meter. The flow of diluent is subtracted from the flow of diluted exhaust gas to determine the flow of the exhaust gas sample into the exhaust sampling system. The diluted exhaust gas is used, for example, to determine the amount of particulate matter within the exhaust gas. The total particulate matter produced by the vehicle's engine can be determined using the calculated flow of the exhaust gas sample into the exhaust sampling system relative to a total flow of exhaust gas produced by the vehicle's engine, which may be measured by another exhaust measurement device.
It may be desirable to obtain dilution ratios greater than 50:1. However, this is difficult to achieve using a single dilution gas flow device since the turn down ratio and resolution of the dilution gas flow device must be very high. The resolution limits the accuracy of the sample flow measured by the dilution gas flow device. That is, the dilution gas flow device must control and measure very small changes in flow it provides. To this end, exhaust sampling systems have been developed that utilize two dilution gas flow devices. In this type of system, one of the dilution gas flow devices measures diluent, which is then mixed with the exhaust gas sample. Subsequent to mixing of the diluent and the exhaust gas sample, the diluted exhaust gas is further diluted by introducing additional diluent from another dilution gas flow device. In this prior art system, the flow of the sample exhaust gas into the exhaust sampling system is measured.
Determination of sample flow by measuring the sample exhaust gas is difficult to achieve accurately due to the high temperature, change in composition and pulsating nature of the exhaust gases. Cooling the exhaust gas to obtain accurate measurement is undesirable since doing so would cause particulate matter to be deposited within the exhaust sampling system prior to its measurement.
What is needed is an exhaust sampling system that does not require a measurement of the exhaust gas sample flow into the exhaust sampling system, and is able to achieve high dilution ratios. Moreover, it is desirable to provide an exhaust sampling system that is easy to calibrate.